This invention relates generally to the art of platinum resistance thermometers and more specifically to methods of maintaining the accuracy of platinum resistance thermometers used in critical temperature measuring applications.
Platinum resistance thermometers (PRT's) are used extensively in scientific and industrial applications when very precise temperature measurements are required. The measurement method depends on an established relationship between the electrical resistance of a very pure, annealed, platinum resistance element and the element's temperature. If the sensing element of the PRT becomes chemically contaminated or strained or work hardened due to differential expansion induced by abnormal temperature transients or by vibrations, the PRT becomes decalibrated. The PRT must be removed and replaced or recalibrated, both of which are expensive operations. Further, the user is normally not aware that decalibration has occured.
In nuclear reactor temperature measuring applications, for example, PRT's are used in which the sensitive platinum resistor element is formed by helically winding a length of platinum resistor wire which is then annealed in an annealing furnace and inserted in a loose fit arrangement into a U-shaped, circular cross-section channel of a ceramic insulator. A pair of low resistance platinum wires are then attached to each end of the resistor coil which extend through a long metal sheath packed with insulating material to an accessable terminal box located outside the reactor core for connection to appropriate instrumentation. These PRT's are precisely calibrated for temperature measurements over a range of typically 020 C. to 350.degree. C. at accuracies within .+-.0.07.degree. C. When these PRT's become decalibrated it is the usual practice to remove the device and replace it with a new or recalibrated PRT. This operation requires that the reactor be shutdown for removal of the PRT and subsequent decontamination of the removed device. This is a very expensive and time consuming process.
Thus, there is a need for a process which would allow in situ restoration of platinum resistance thermometers to their original calibration to reduce maintenance cost and lost production time for processes using these thermometers.